Semiconductor light emitting device and method for fabricating the same

ABSTRACT

A semiconductor light emitting device includes a semiconductor multilayer structure comprising a plurality of Group III–V nitride semiconductor layers including two semiconductor layers of different conductivity types, and a transparent electrode formed on the semiconductor multilayer structure. The transparent electrode contains an impurity element developing the same conductivity type as that of an impurity element introduced into a semiconductor in the semiconductor multilayer structure, which semiconductor has an interface with the transparent electrode. Therefore, contact resistance between the transparent electrode and the semiconductor having the interface with the transparent electrode is decreased.

BACKGROUND OF THE INVENTION

The present invention relates to semiconductor light emitting devicesmade of Group III–V nitride semiconductors, which are capable ofemitting light in the blue to ultraviolet regions.

Recently, light emitting diodes (GaN-based LEDs), using a Group III–Vnitride (hereinafter, referred to simply as a “nitride”) expressed by ageneral formula B_(z) ,Al_(x)Ga_(l−x−y−z)In_(y)N, where 0≦x≦1, 0≦y≦1,0≦z≦1, and x+y+z=1, have found wide application in various kinds ofdisplay panels, large display apparatus and traffic lights, for example.White LEDs, in which a GaN-based LED is combined with a fluorescentsubstance, have also been put into practical use, and are expected toreplace the currently used lighting equipment, if their luminousefficiency is improved in the future.

FIG. 18 illustrates a cross-sectional structure of a known blue lightemitting diode in which nitride semiconductors are used (See JapaneseLaid-Open Publication Nos. 07-094782, 10-173224, and 2000-5891.) Asshown in FIG. 18, in the known blue light emitting diode, a firstsemiconductor layer 102 made of an n-type nitride semiconductor, and asecond semiconductor layer 103 made of a p-type nitride semiconductorare sequentially stacked on a substrate 101 made of sapphire.

A first electrode 104 made of nickel and gold with a thickness of fromabout 2 nm to about 5 nm is formed on the second semiconductor layer103. The first electrode 104 can make a good ohmic contact with thep-type nitride semiconductor.

A second electrode 105 made of gold is formed on the first electrode104. The second electrode 105, which is for wire bonding, passes throughthe first electrode 104 to reach the second semiconductor layer 103. Ann-type ohmic electrode 106 is formed on an exposed portion of the firstsemiconductor layer 102.

With this structure, in the known blue light emitting diode,recombination radiation (generated light), emitted by the pn junctionformed by the interface between the first and second semiconductorlayers 102 and 103, is transmitted through the second semiconductorlayer 103 and the first electrode 104, and then extracted.

However, a problem with the known blue light emitting diode is that therecombination radiation produced by the pn junction is partiallyabsorbed by the first electrode 104 made of the metals. To deal withthis problem, if the thickness of the first electrode 104 is reducedsignificantly, the amount of radiation transmitting through the firstelectrode 104 can be increased. In that case, however, a trade-offoccurs in which series resistance (sheet resistance) in the firstelectrode 104 is increased, which makes it difficult to significantlyincrease the optical electric characteristics of the device, that is,the device characteristics.

Alternatively, instead of the metals, transparent material may be usedto form the first electrode 104 in order to increase thelight-extraction efficiency. Nevertheless, a problem also arises in thiscase, in which contact resistance between the p-type nitridesemiconductor layer and the transparent electrode formed thereon islarge.

In addition, there is another problem in that nitride semiconductors, inwhich the activation ratio of an impurity, particularly of a p-typeimpurity, that determines the conductivity type of the semiconductor issmall, have large sheet resistance.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, it is therefore an object ofthe present invention to decrease contact resistance with respect to atransparent electrode in a nitride semiconductor device.

In order to achieve the above object, an inventive semiconductor lightemitting device employs a structure in which a transparent electrodethat has an interface with a nitride semiconductor layer is doped withan impurity developing the same conductivity type as that of an impurityintroduced into the nitride semiconductor layer, or is doped with ametal that can adsorb hydrogen.

Further, in order to achieve the above object, an inventivesemiconductor light emitting device employs a structure in which apassivation film that has an interface with a nitride semiconductorlayer is doped with an impurity developing the same conductivity type asthat of an impurity introduced into the nitride semiconductor layer, oris doped with a metal that can adsorb hydrogen.

Specifically, a first inventive semiconductor light emitting deviceincludes a semiconductor multilayer structure comprising a plurality ofGroup III–V nitride semiconductor layers including two semiconductorlayers of different conductivity types, and a transparent electrodeformed on the semiconductor multilayer structure. The transparentelectrode contains an impurity element developing the same conductivitytype as that of an impurity element introduced into a semiconductor inthe semiconductor multilayer structure. The semiconductor has aninterface with the transparent electrode.

In the first inventive semiconductor light emitting device, the impurityelement contained in the transparent electrode is diffused into thesemiconductor layer having the interface with the transparent electrodeby a heat treatment performed during the fabrication process, so thatthe impurity element diffused from the transparent electrode into thesemiconductor layer causes a decrease in the value of resistance in thesemiconductor layer where the semiconductor layer is near the interfacewith the transparent electrode. Thus, the contact resistance of thesemiconductor layer with respect to the transparent electrode isdecreased.

A second inventive semiconductor light emitting device includes asemiconductor multilayer structure comprising a plurality of Group III–Vnitride semiconductor layers including two semiconductor layers ofdifferent conductivity types, and a transparent electrode formed on thesemiconductor multilayer structure. The transparent electrode contains ametal element that adsorbs hydrogen.

In the second inventive semiconductor light emitting device, when themetal element that adsorbs hydrogen, contained in the transparentelectrode, is diffused into a semiconductor layer having an interfacewith the transparent electrode by a heat treatment performed during thefabrication process, the metal element that adsorbs hydrogen, diffusedfrom the transparent electrode into the semiconductor layer, adsorbs(binds to) the hydrogen atoms that have been bound to an impurityelement introduced into the semiconductor layer. This increases theactivation ratio of the impurity element introduced into thesemiconductor layer where the semiconductor layer is near the interfacewith the transparent electrode, resulting in a decrease in theresistance of the semiconductor layer. Accordingly, the contactresistance of the semiconductor layer with respect to the transparentelectrode is reduced.

A third inventive semiconductor light emitting device includes asemiconductor multilayer structure comprising a plurality of Group III–Vnitride semiconductor layers including two semiconductor layers ofdifferent conductivity types, and a passivation film formed on thesemiconductor multilayer structure. The passivation film contains animpurity element developing the same conductivity type as that of animpurity element introduced into a semiconductor in the semiconductormultilayer structure. The semiconductor has an interface with thepassivation film.

In the third inventive semiconductor light emitting device, the impurityelement contained in the passivation film is diffused into thesemiconductor layer having the interface with the passivation film by aheat treatment performed during the fabrication process, so that theimpurity element diffused into the semiconductor layer from thepassivation film causes a reduction in the value of resistance in thesemiconductor layer where the semiconductor layer is near the interfacewith the passivation film. As a result, the value of resistance (sheetresistance) in the upper portion of the semiconductor layer is allowedto be small.

A fourth inventive semiconductor light emitting device includes asemiconductor multilayer structure comprising a plurality of Group III–Vnitride semiconductor layers including two semiconductor layers ofdifferent conductivity types, and a passivation film formed on thesemiconductor multilayer structure. The passivation film contains ametal element that adsorbs hydrogen.

In the fourth inventive semiconductor light emitting device, when themetal element contained in the passivation film is diffused into asemiconductor layer having an interface with the passivation film by aheat treatment performed during the fabrication process, the metalelement that adsorbs hydrogen, diffused from the passivation film intothe semiconductor layer, adsorbs (binds to) the hydrogen atoms that havebeen bound to an impurity element introduced into the semiconductorlayer. This increases the activation ratio of the impurity elementintroduced into the semiconductor layer where the semiconductor layer isnear the interface with the passivation film, resulting in a decrease inthe value of resistance in the semiconductor layer. Accordingly, thevalue of resistance (sheet resistance) in the upper portion of thesemiconductor layer is reduced.

In the first or third inventive semiconductor light emitting device, theimpurity elements are preferably magnesium, zinc, beryllium, or silicon.

In the second or fourth inventive semiconductor light emitting device,the metal element is preferably nickel, palladium, or platinum.

The third or fourth inventive semiconductor light emitting devicepreferably further includes a transparent electrode formed on thesemiconductor multilayer structure where the passivation film is notformed.

In the first through fourth inventive semiconductor light emittingdevices, the transparent electrode is preferably made of indium tinoxide or gallium oxide.

Further, the first through fourth inventive semiconductor light emittingdevices preferably further include, on the transparent electrode, amultilayer film that reflects light emitted from the semiconductormultilayer structure, and includes a plurality of dielectric layers.

The first through fourth inventive semiconductor light emitting devicespreferably further include a multilayer film, which is formed to theside of the semiconductor multilayer structure opposite to thetransparent electrode, and which reflects light emitted from thesemiconductor multilayer structure, and includes a plurality ofdielectric layers or a plurality of semiconductor layers.

In this case, the multilayer film is preferably made of at least twosubstances among silicon oxide, silicon nitride, niobium oxide, hafniumoxide, titanium oxide and tantalum oxide.

A first inventive method for fabricating a semiconductor light emittingdevice includes the steps of forming, on a substrate, a semiconductormultilayer structure comprising a plurality of Group III–V nitridesemiconductor layers including two semiconductor layers of differentconductivity types; and forming a transparent electrode on anelectrode-formation face of the semiconductor multilayer structure byusing material that contains an impurity element developing the sameconductivity type as that of an impurity element introduced into asemiconductor having the electrode-formation face, and thenheat-treating the transparent electrode.

In accordance with the first inventivesemiconductor-light-emitting-device fabrication method, the impurityelement contained in the transparent electrode formed on thesemiconductor multilayer structure is diffused into the semiconductorlayer having the interface with the transparent electrode by the heattreatment. The impurity element diffused from the transparent electrodeinto the semiconductor layer causes a reduction in the value ofresistance in the semiconductor layer where the semiconductor layer isnear the interface with the transparent electrode. Accordingly, thecontact resistance of the semiconductor layer with respect to thetransparent electrode is permitted to be small.

A second method for fabricating a semiconductor light emitting deviceincludes the steps of forming, on a substrate, a semiconductormultilayer structure comprising a plurality of Group III–V nitridesemiconductor layers including two semiconductor layers of differentconductivity types; and forming a transparent electrode on thesemiconductor multilayer structure by using material that contains ametal element that adsorbs hydrogen, and then heat-treating thetransparent electrode.

According to the second inventive semiconductor-light-emitting-devicefabrication method, the metal element that adsorbs hydrogen, containedin the transparent electrode formed on the semiconductor multilayerstructure, is diffused into a semiconductor layer having an interfacewith the transparent electrode by the heat treatment. The metal elementthat adsorbs hydrogen, diffused from the transparent electrode into thesemiconductor layer, adsorbs (binds to) the hydrogen atoms that havebeen bound to an impurity element introduced into the semiconductorlayer. This increases the activation ratio of the impurity elementintroduced into the semiconductor layer where the semiconductor layer isnear the interface with the transparent electrode, leading to a decreasein the resistance of the semiconductor layer. As a result, the contactresistance of the semiconductor layer with respect to the transparentelectrode is reduced.

The first or second inventive semiconductor-light-emitting-devicefabrication method preferably further includes, before thetransparent-electrode formation step, the steps of: forming apassivation film on the semiconductor multilayer structure, and removingfrom the passivation film a portion in which the transparent electrodeis to be formed. The passivation film is preferably formed usingmaterial that contains an impurity element developing the sameconductivity type as that of an impurity element introduced into asemiconductor in the semiconductor multilayer structure. Thesemiconductor has an interface with the passivation film.

The first or second inventive semiconductor-light-emitting-devicefabrication method preferably further includes, before thetransparent-electrode formation step, the steps of: forming apassivation film on the semiconductor multilayer structure, and removingfrom the passivation film a portion in which the transparent electrodeis to be formed. The passivation film is preferably formed usingmaterial that contains a metal element that adsorbs hydrogen.

A third inventive method for fabricating a semiconductor light emittingdevice includes the steps of: forming, on a substrate, a semiconductormultilayer structure comprising a plurality of Group III–V nitridesemiconductor layers including two semiconductor layers of differentconductivity types; forming a first electrode made of metal on thesemiconductor multilayer structure; removing the substrate from thesemiconductor multilayer structure; and forming a transparent electrodeon a second-electrode-formation face of the semiconductor multilayerstructure by using material that contains an impurity element developingthe same conductivity type as that of an impurity element introducedinto a semiconductor having the second-electrode-formation face, whereinthe second-electrode-formation face opposes the first electrode, andthen heat-treating the transparent electrode.

In accordance with the third inventivesemiconductor-light-emitting-device fabrication method, even in a casewhere unconductive insulative material such as sapphire is used as thematerial for the substrate, it is possible to respectively form a firstelectrode and a second electrode on the upper and lower faces of thesemiconductor multilayer structure so that the first and secondelectrodes oppose each other. In addition, the impurity element diffusedfrom the transparent electrode into the semiconductor layer having theinterface with the transparent electrode causes a reduction in the valueof resistance in the semiconductor layer where the semiconductor layeris in the vicinity of the interface with the transparent electrode.Accordingly, the contact resistance of the semiconductor layer withrespect to the transparent electrode is allowed to be small.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a blue light emitting diode in accordancewith a first embodiment of the present invention. FIG. 1A illustratesthe plan configuration, while FIG. 1B illustrates the cross-sectionalview taken along the line Ib—Ib of FIG. 1A.

FIGS. 2A through 2C are cross-sectional views illustrating sequentialprocess steps for fabricating the blue light emitting diode inaccordance with the first embodiment of the present invention.

FIGS. 3A and 3B are cross-sectional views illustrating sequentialprocess steps for fabricating the blue light emitting diode inaccordance with the first embodiment of the present invention.

FIG. 4 is a cross-sectional view of a blue light emitting diode inaccordance with a second modified example of the first embodiment of thepresent invention.

FIG. 5 is a cross-sectional view of an ultraviolet light emitting diodein accordance with a second embodiment of the present invention.

FIGS. 6A through 6C are cross-sectional views illustrating sequentialprocess steps for fabricating the ultraviolet light emitting diode inaccordance with the second embodiment of the present invention.

FIGS. 7A through 7C are cross-sectional views illustrating sequentialprocess steps for fabricating the ultraviolet light emitting diode inaccordance with the second embodiment of the present invention.

FIG. 8 is a cross-sectional view of a blue light emitting diode inaccordance with a third embodiment of the present invention.

FIGS. 9A through 9C are cross-sectional views illustrating sequentialprocess steps for fabricating the blue light emitting diode inaccordance with the third embodiment of the present invention.

FIGS. 10A through 10C are cross-sectional views illustrating sequentialprocess steps for fabricating the blue light emitting diode inaccordance with the third embodiment of the present invention.

FIG. 11 is a cross-sectional view of a blue light emitting diode inaccordance with a second modified example of the third embodiment of thepresent invention.

FIG. 12 is a cross-sectional view of a blue light emitting diode inaccordance with a fourth embodiment of the present invention.

FIGS. 13A through 13D are cross-sectional views illustrating sequentialprocess steps for fabricating the blue light emitting diode inaccordance with the fourth embodiment of the present invention.

FIGS. 14A through 14C are cross-sectional views illustrating sequentialprocess steps for fabricating the blue light emitting diode inaccordance with the fourth embodiment of the present invention.

FIG. 15 is a cross-sectional view of a blue-light surface-emitting laserdevice in accordance with a fifth embodiment of the present invention.

FIGS. 16A through 16C are cross-sectional views illustrating sequentialprocess steps for fabricating the blue-light surface-emitting laserdevice in accordance with the fifth embodiment of the present invention.

FIGS. 17A through 17C are cross-sectional views illustrating sequentialprocess steps for fabricating the blue-light surface-emitting laserdevice in accordance with the fifth embodiment of the present invention.

FIG. 18 is a cross-sectional view of a known blue light emitting diode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

Hereinafter, a first embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIGS. 1A and 1B illustrate a blue light emitting diode in accordancewith the first embodiment of the present invention. FIG. 1A illustratesthe plan configuration, while FIG. 1B illustrates the cross-sectionalstructure taken along the line Ib—Ib of FIG. 1A.

As shown in FIGS. 1A and 1B, a first semiconductor layer 12, amulti-quantum well (MQW) active layer 13, and a second semiconductorlayer 14 are sequentially formed on a substrate 11 made of sapphire, forexample. The first semiconductor layer 12 is made of n-type galliumnitride (GaN) having a thickness of about 4 μm and a carrier density ofabout 1×10¹⁷ cm². The MQW active layer 13 is formed by stacking threepairs of an about 7 nm thick barrier layer of GaN and an about 3 nmthick well layer of In_(0.3)Ga_(0.7)N. The second semiconductor layer 14is made of p-type gallium nitride (GaN) having a thickness of about 0.8μm and a carrier density of about 1×10¹⁸ cm².

A transparent electrode 15 having a thickness of about 100 nm and madeof indium tin oxide (ITO) is formed on the second semiconductor layer14.

A bonding pad 16 of gold (Au) is formed selectively on the transparentelectrode 15, and an n-type ohmic electrode 17, made of a multilayerstructure of titanium (Ti) and gold (Au), is formed on a selectivelyexposed portion of the first semiconductor layer 12.

By this structure, blue light, generated and emitted from the MQW activelayer 13, and passing through the second semiconductor layer 14 and thetransparent electrode 15, is taken out to the exterior.

The first embodiment is characterized in that the impurity elementintroduced into the ITO that forms the transparent electrode 15 ismagnesium (Mg), which is the impurity element introduced into the p-typesecond semiconductor layer 14. As will be described later, the magnesiumintroduced into the ITO is diffused into the second semiconductor layer14 by an annealing performed during fabrication process, causing contactresistance between the second semiconductor layer 14 and the transparentelectrode 15 to decrease.

It should be noted that the impurity element to introduce into thetransparent electrode 15 is not limited to magnesium, but zinc (Zn),beryllium (Be), or any other dopant that makes the conductivity type ofgallium nitride be p-type, may be used.

In addition, instead of the MQW active layer 13, a single-quantum well(SQW) active layer of indium gallium nitride with a thickness of about20 nm may be provided.

Moreover, as shown in FIG. 1A, since the transparent electrode 15 isconductive, the location and shape of the electrode may be determinedarbitrarily.

As described above, in the first embodiment, magnesium, which is theimpurity element introduced into the p-type second semiconductor layer14, is introduced into the transparent electrode 15. Therefore, contactresistance between the second semiconductor layer 14 and the transparentelectrode 15 is decreased, thereby allowing the operating voltage to bereduced.

Hereinafter, referring to the accompanying drawings, it will bedescribed how to fabricate a blue light emitting diode having theabove-mentioned structure.

FIGS. 2A through 2C and FIGS. 3A and 3B are cross-sectional viewsillustrating sequential process steps for fabricating a blue lightemitting diode in accordance with the first embodiment of the presentinvention.

First, as shown in FIG. 2A, a low-temperature buffer layer (not shown)made of gallium nitride is grown on a substrate 11 made of sapphire by ametal organic vapor phase epitaxy (MOVPE) process. The substrate 11 isabout 5.1 cm (=2 inches) in diameter, and the plane orientation of theprincipal surface thereof is a (0001) plane. In the MOVPE process,trimethylgallium (TMG) is used as a gallium source, ammonia (NH₃) isused as a nitrogen source, and hydrogen (H₂) is used as a carrier gas,while the growth temperature is set at about 500° C. The buffer layerbuffers a lattice mismatch between the sapphire and a firstsemiconductor layer 12, for example, grown on the sapphire.Subsequently, while mono-silane (SiH₄), which is a source materialcontaining silicon serving as a donor impurity, is introduced, and withthe growth temperature being set to about 1030° C., the firstsemiconductor layer 12 made of n-type gallium nitride with a thicknessof about 4 μm is grown on the low-temperature buffer layer. Then, thesupply of the mono-silane is stopped, and a barrier layer made ofgallium nitride with a thickness of about 7 nm is grown on the firstsemiconductor layer 12. The carrier gas is then changed to nitrogen(N₂), and at the same time the growth temperature is lowered to about800° C., and while trimethylindium (TMI) as an indium source is alsosupplied, a well layer is grown on the barrier layer. The well layer hasa thickness of about 3 nm, and is made of indium gallium nitride, inwhich indium proportion is 30%. The barrier layer and the well layer aregrown alternately in three pairs, thereby forming a MQW active layer 13.By this quantum well structure, the MQW active layer 13 generates bluelight with a wavelength of about 470 nm. As mentioned above, when thebarrier layers of gallium nitride are grown, hydrogen is used as thecarrier gas, and the growth temperature is set at about 1030° C. On theother hand, when the well layers of indium gallium nitride are grown,nitrogen is used as the carrier gas, and the growth temperature is setat about 800° C.

Next, cyclopentadienyl magnesium (Cp₂Mg), which is a source materialcontaining magnesium as an acceptor impurity, is introduced into therespective source gases of trimethylgallium and ammonia, and a secondsemiconductor layer 14 made of p-type gallium nitride with a thicknessof about 0.8 μm is grown on the MQW active layer 13. After the secondsemiconductor layer 14 has been grown, the second semiconductor layer 14is subjected to an annealing process performed using an annealingfurnace for 20 minutes in a nitrogen ambient at a temperature of about750° C. Through the annealing process, the p-type dopant introduced intothe second semiconductor layer 14 is activated, which further reducesthe resistance of the second semiconductor layer 14.

Subsequently, as shown in FIG. 2B, the second semiconductor layer 14,the MQW active layer 13, and upper portions of the first semiconductorlayer 12 are removed selectively by dry etching, such as reactive ionetching (RIE) using, e.g., chlorine (Cl₂) as an etching gas, orinductively coupled plasma (ICP) etching, thereby forming n-typeelectrode formation regions 12 a in the first semiconductor layer 12.

Then, as shown in FIG. 2C, ITO, into which magnesium, i.e., the sameimpurity element as the p-type dopant in the second semiconductor layer14, has been introduced, is selectively grown to a thickness of about100 nm on the second semiconductor layers 14, thereby formingtransparent electrodes 15. The ITO may be grown by a sputtering process,a pulsed laser deposition (PLD) method, an electron beam (EB) process,or a sol-gel method, for example. In terms of reducing the resistance ofthe ITO, a sputtering process or a PLD method is preferable. Further, ifa sputtering process or a PLD method is employed, a target material isnormally formed by sintering, thus making it easy to introduce animpurity element such as magnesium. Subsequently, after the transparentelectrodes 15 have been grown, the transparent electrodes 15 aresubjected to an annealing process performed at a temperature of about500° C. Through the annealing process, part of the magnesium introducedinto the ITO is diffused into the second semiconductor layers 14 throughthe interfaces between the ITO and the second semiconductor layers 14.This leads to a decrease in the value of resistance in the secondsemiconductor layers 14 where the second semiconductor layers 14 arenear the interfaces with the transparent electrodes 15, such that thetransparent electrodes 15 having small contact resistance with respectto the second semiconductor layers 14 are formed. In this embodiment,the impurity element to introduce into the ITO is not limited tomagnesium, but zinc or beryllium may be used. Nevertheless, magnesium ispreferable in terms of activation of the dopant. Further, thetransparent electrodes 15 are not limited to ITO, but may be made of anysubstance that makes the transparent electrodes 15 transparent withrespect to emitted light having a wavelength of 470 nm, and tin oxide(SnO₂) or zinc oxide (ZnO), for example, may be used.

Then, as shown in FIG. 3A, bonding pads 16 for wire bonding areselectively formed on the respective transparent electrodes 15.Subsequently, titanium and gold are sequentially grown on the n-typeelectrode formation regions 12 a in the first semiconductor layer 12,thereby forming n-type ohmic electrodes 17.

Next, as shown in FIG. 3B, the substrate 11 is divided into chips eachabout 300 μm square, thereby obtaining blue light emitting diodes.

In this manner, magnesium, that is, the impurity element with which thep-type second semiconductor layers 14 have been doped, is introducedbeforehand into the transparent electrodes 15 (p-type electrodes), anddiffused through the interfaces into the second semiconductor layers 14by an annealing process. Therefore, in the resultant blue light emittingdiode, in which emitted light is taken out through the p-type secondsemiconductor layer 14, the contact resistance of the transparentelectrode 15 with respect to the second semiconductor layer 14 isallowed to be small, so that the second semiconductor layer 14 has smallresistance near its interface with the transparent electrode 15. As aresult, it is possible to lower the operating voltage.

First Modified Example of the First Embodiment

In the first embodiment, a dopant that makes the conductivity type ofgallium nitride be p-type is introduced into the transparent material(ITO) that forms the transparent electrodes 15. However, in addition tothe p-type dopant, a metal element that tends to adsorb (bind to)hydrogen atoms, e.g., nickel (Ni), palladium (Pd), or platinum (Pt), maybe introduced. In that case, in order to form the transparent electrodes15, a metal element, such as nickel, that easily adsorbs hydrogen atomsmay be introduced beforehand into the target material for growing thetransparent electrodes 15.

Normally, a p-type dopant introduced into a p-type gallium nitridesemiconductor tends to bind to hydrogen atoms, causing the p-type dopantto be deactivated. In view of this, if metal atoms that easily adsorbhydrogen atoms are diffused into the p-type semiconductor layer throughthe transparent electrode 15, the metal atoms diffused into the p-typesemiconductor layer attract the hydrogen atoms that have been taken intothe p-type semiconductor layer.

In this manner, the metal atoms such as nickel atoms separate thehydrogen atoms that cause the p-type dopant to be deactivated, from thep-type dopant, so that activation of the p-type dopant such as magnesiumis facilitated. Therefore, the p-type second semiconductor layer 14 hassmall resistance where the p-type second semiconductor layer 14 is inthe vicinity of the interface with the transparent electrode 15. As aresult, it is possible to form the transparent electrode 15 with smallcontact resistance with respect to the second semiconductor layer 14.

Second Modified Example of the First Embodiment

In the first embodiment, generated light is extracted through the p-typesecond semiconductor layer 14. However, a flip-chip device, in whichgenerated light is taken out through a substrate 11, may be formed.

As shown in FIG. 4, in a blue light emitting diode, a transparentelectrode 15 is secured onto a mounting substrate 20 with ahigh-reflectance film 21 and a first solder material 22 being interposedtherebetween. The high-reflectance film 21 is made of a multilayer filmcomposed of a plurality of dielectrics. Further, an n-type ohmicelectrode 17 is secured onto the mounting substrate 20 with a secondsolder material 23 being interposed therebetween.

In this way, in the blue light emitting diode in accordance with asecond modified example, the high-reflectance film 21, instead of thebonding pad 16, is provided on the transparent electrode 15, so thatlight emitted toward the transparent electrode 15 is reflected by thehigh-reflectance film 21, and extracted through the substrate 11. Inthis modified example, the higher the reflectance of thehigh-reflectance film 21 the better, and it is preferable that thereflectance is at least 70% or more.

(Second Embodiment)

Hereinafter, a second embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 5 illustrates a cross-sectional structure of an ultraviolet lightemitting diode in accordance with the second embodiment of the presentinvention. In FIG. 5, the same members as those shown in FIGS. 1A and 1Bare identified by the same reference numerals and the descriptionthereof will be omitted herein.

As shown in FIG. 5, a first semiconductor layer 32, a MQW active layer33, and a second semiconductor layer 34 are sequentially formed on asubstrate 11 made of sapphire, for example. The first semiconductorlayer 32 is made of n-type aluminum gallium nitride (Al_(0.4)Ga_(0.6)N)having a thickness of about 4 μm and a carrier density of about 1×10¹⁷cm². The MQW active layer 33 is formed by stacking three pairs of anabout 7 nm thick barrier layer of aluminum gallium nitride(Al_(0.12)Ga_(0.88)N) and an about 3 nm thick well layer of GaN. Thesecond semiconductor layer 34 is made of p-type aluminum gallium nitride(Al_(0.4)Ga_(0.6)N) having a thickness of about 0.8 μm and a carrierdensity of about 1×10¹⁸ cm².

An n-type ohmic electrode 37 made of titanium and aluminum is formed onan exposed portion of the first semiconductor layer 32.

On the second semiconductor layer 34, formed is a transparent electrode35 having a thickness of about 100 nm and made of gallium oxide (Ga₂O₃),into which about 1 mol % of tin (Sn) has been introduced. It ispreferable that β-(cubic) gallium oxide be used, in which case theconductivity becomes excellent. Further, the impurity element introducedinto the gallium oxide that forms the transparent electrode 35 ismagnesium (Mg), which is the impurity element introduced into the p-typesecond semiconductor layer 34. As in the first embodiment, the magnesiumintroduced into the gallium oxide is diffused into the secondsemiconductor layer 34 through an annealing performed during fabricationprocess, causing contact resistance between the second semiconductorlayer 34 and the transparent electrode 35 to be decreased.

ITO, normally used as a transparent electrode, has low transmissivitywith respect to ultraviolet light with a wavelength of about 300 nm, andthus is not suitable for a transparent electrode. On the other hand,gallium oxide, particularly β-gallium oxide, into which tin oxide hasbeen introduced, has high transmissivity with respect to ultravioletlight in the 300 nm wavelength range, and is thus suitable for thetransparent electrode 35 formed in the ultraviolet light emitting diodein accordance with the second embodiment.

As described above, in the ultraviolet light emitting diode of thesecond embodiment, since the transparent electrode 35 with hightransmissivity is used, light-extraction efficiency is increased. Inaddition, the impurity developing the same conductivity type as that ofthe impurity that makes the second semiconductor layer 34 be of p-type,is introduced into the transparent electrode 35, such that contactresistance between the transparent electrode 35 and the secondsemiconductor layer 34 is small, allowing the operating voltage to bedecreased.

It should be noted that the impurity element to introduce into thetransparent electrode 35 is not limited to magnesium, but zinc,beryllium, or any other dopant that renders the conductivity of galliumnitride p-type may be used.

Moreover, instead of the MQW active layer 33, a single-quantum well(SQW) active layer of gallium nitride with a thickness of about 20 nmmay be provided.

Hereinafter, referring to the accompanying drawings, it will bedescribed how to fabricate an ultraviolet light emitting diode havingthe above-mentioned structure.

FIGS. 6A through 6C and FIGS. 7A through 7C are cross-sectional viewsillustrating sequential process steps for fabricating an ultravioletlight emitting diode in accordance with the second embodiment of thepresent invention.

First, as shown in FIG. 6A, a low-temperature buffer layer (not shown)of aluminum gallium nitride is grown on a substrate 11 made of sapphireby a MOVPE process. The substrate 11 is about 5.1 cm in diameter, andthe plane orientation of the principal surface thereof is a (0001)plane. In the MOVPE process, trimethylgallium is used as a galliumsource, trimethylaluminum is used as an aluminum source, ammonia is usedas a nitrogen source, and hydrogen is used as a carrier gas, while thegrowth temperature is set at about 500° C. The low-temperature bufferlayer buffers a lattice mismatch between the sapphire and a firstsemiconductor layer 32, for example, grown on the sapphire. In thisprocess step, the low-temperature buffer layer may be made of galliumnitride. Subsequently, while mono-silane, which is a source materialcontaining silicon as a donor impurity, is introduced, and with thegrowth temperature being set at about 1030° C., the first semiconductorlayer 32 made of n-type aluminum gallium nitride having a thickness ofabout 4 μm is grown on the low-temperature buffer layer. Then, thesupply of the mono-silane is stopped, and a barrier layer made ofaluminum gallium nitride with a thickness of about 7 nm is grown on thefirst semiconductor layer 32. Thereafter, the supply of thetrimethylaluminum as the aluminum source is stopped, and a well layermade of gallium nitride having a thickness of about 3 nm is grown on thebarrier layer. The barrier layer and the well layer are grownalternately in three pairs, thereby forming a MQW active layer 33. Bythis quantum well structure, the MQW active layer 33 generatesultraviolet light with a wavelength of about 360 nm. Then,cyclopentadienyl magnesium, which is a source material containingmagnesium as an acceptor impurity, is introduced into the respectivesource gases of trimethylgallium, trimethylaluminum, and ammonia, and asecond semiconductor layer 34 made of p-type aluminum gallium nitridewith a thickness of about 0.8 μm is grown on the MQW active layer 33.

Next, as shown in FIG. 6B, β-gallium oxide is grown to a film thicknessof about 100 nm on the second semiconductor layer 34 by a PLD method,for example, thereby forming a transparent electrode 35, wherein tin formaking the electrode itself conductive, and magnesium to be diffusedinto the second semiconductor layer 34 have been introduced into theβ-gallium oxide. The gallium oxide film may be grown by a sputteringprocess, but a PLD method, which gives excellent crystallinity, ispreferable. Subsequently, after the transparent electrode 35 has beengrown, the second semiconductor layer 34 and the transparent electrode35 are subjected to an annealing process performed using an annealingfurnace for 20 minutes in a nitrogen ambient at a temperature of about750° C. Through the annealing process, the resistance of the transparentelectrode 35 is decreased, while at the same time the p-type dopant inthe second semiconductor layer 34, including the p-type dopant diffusedfrom the transparent electrode 35, is activated, thereby furtherlowering the resistance of the second semiconductor layer 34.

Next, as shown in FIG. 6C, patterning is performed to selectively removethe transparent electrode 35 where the transparent electrode 35 islocated above n-type ohmic-electrode formation regions.

Subsequently, as shown in FIG. 7A, the second semiconductor layer 34,the MQW active layer 33, and upper portions of the first semiconductorlayer 32 are selectively removed by dry etching, such as RIE using,e.g., chlorine as an etching gas, or ICP etching, thereby forming n-typeelectrode formation regions 32 a in the first semiconductor layer 32.

Next, as shown in FIG. 7B, bonding pads 16 for wire bonding areselectively formed on the respective transparent electrodes 35. Titaniumand aluminum are then sequentially grown on the n-type electrodeformation regions 32 a in the first semiconductor layer 32, therebyforming n-type ohmic electrodes 37.

Next, as shown in FIG. 7C, the substrate 11 is divided into chips eachabout 300 μm square, thereby obtaining ultraviolet light emittingdiodes.

As described above, in the fabrication method of the second embodiment,since tin-oxide-added gallium oxide having high transmissivity withrespect to ultraviolet light is used to form the transparent electrode35, the light-extraction efficiency is extremely favorable, resulting inan increase in power conversion efficiency.

Additionally, since magnesium, that is, the p-type impurity element withwhich the p-type second semiconductor layer 34 has been doped, isintroduced into the transparent electrode 35, part of the magnesium isdiffused into the second semiconductor layer 34 where the secondsemiconductor layer 34 is in the vicinity of the interface with thetransparent electrode 35 by an annealing process performed after theformation of the transparent electrode 35 as in the first embodiment.This allows the second semiconductor layer 34 to have small resistancenear the interface with the transparent electrode 35, thereby reducingcontact resistance between the second semiconductor layer 34 and thetransparent electrode 35.

It should be noted that the impurity element to introduce into galliumoxide is not limited to magnesium, but zinc, beryllium, or any otherdopant that makes aluminum gallium nitride develop p-type conductivity,may be used.

Modified Example of the Second Embodiment

In the second embodiment, a dopant that renders the conductivity ofaluminum gallium nitride p-type is introduced into the transparentmaterial (gallium oxide) that forms the transparent electrodes 35.However, in addition to the p-type dopant, a metal element that tends toadsorb (bind to) hydrogen atoms, e.g., nickel (Ni), palladium (Pd), orplatinum (Pt), may be introduced. In that case, in order to form thetransparent electrodes 35, a metal element, such as nickel, that tendsto adsorb hydrogen atoms may be introduced beforehand into the targetmaterial for growing the transparent electrodes 35.

Then, the ultraviolet light emitting diode made of nitridesemiconductors in accordance with this modified example achieves anincrease in the light-extraction efficiency as well as a decrease in theoperating voltage.

(Third Embodiment)

Hereinafter, a third embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 8 illustrates a cross-sectional structure of a blue light emittingdiode in accordance with the third embodiment of the present invention.In FIG. 8, the same members as those shown in FIGS. 1A and 1B areidentified by the same reference numerals and the description thereofwill be omitted herein.

As shown in FIG. 8, the blue light emitting diode in accordance with thethird embodiment has a so-called N-up structure, in which a transparentelectrode 45 is formed on a first semiconductor layer 12 made of n-typegallium nitride.

A second semiconductor layer 14 made of p-type gallium nitride is formedto the side of a MQW active layer 13 opposite to the first semiconductorlayer 12, that is, the second semiconductor layer 14 is formed under theMQW active layer 13.

A p-type electrode 41 made of platinum having a thickness of about 100nm is formed underneath the second semiconductor layer 14. Underneaththe p-type electrode 41, a plated underlying layer 42 made of gold witha thickness of about 200 nm is formed. Underneath the plated underlyinglayer 42, a plated layer 43 made of gold having a thickness of about 50μm is formed.

The third embodiment is characterized in that the impurity elementintroduced into the ITO that forms the transparent electrode 45 issilicon (Si), which is the impurity element introduced into the n-typefirst semiconductor layer 12. As will be described later, the siliconintroduced into the ITO is diffused into the first semiconductor layer12 through an annealing performed during fabrication process, so thatcontact resistance between the first semiconductor layer 12 and thetransparent electrode 45 decreases.

It should be noted that the impurity element to introduce into thetransparent electrode 45 is not limited to silicon, but a dopant, suchas germanium (Ge), that renders the conductivity of gallium nitriden-type, may be used.

Further, in this embodiment, the substrate 11 of sapphire is removedfrom the semiconductor multilayer structure, and the plated layer 43 ofgold is provided instead. Therefore, instead of the sapphire havinginferior heat-dispersion characteristics, the plated layer 43 withexcellent heat-dispersion characteristics is mounted on a submount, thusensuring that the temperature characteristics of the device increases.

In this embodiment, although the N-up structure is employed as shown inFIG. 8, a so-called P-up structure, in which a transparent electrode isformed on the face of the p-type second semiconductor layer 14 oppositeto the MQW active layer 13, may be adopted. In that case, magnesium isintroduced into the transparent electrode as in the first embodiment.Then, as in the case of the N-up structure, a device having excellentheat-dispersion characteristics, and capable of operating at lowoperating voltage, is obtained.

Moreover, as in the second embodiment, aluminum gallium nitride may beused to form the semiconductor multilayer structure, so that the MQWactive layer emits ultraviolet light. In that case, it is preferablethat the transparent electrode 45 be made of gallium oxide, inparticular, β-gallium oxide, into which tin oxide and an impurity thatdevelops the same conductivity type as that of an impurity introducedinto a semiconductor layer having an interface with the transparentelectrode 45, have been introduced.

Furthermore, instead of the MQW active layer 13, a single-quantum well(SQW) active layer of indium gallium nitride having a thickness of about20 nm may be provided.

Hereinafter, referring to the accompanying drawings, it will bedescribed how to fabricate a blue light emitting diode having theabove-mentioned structure.

FIGS. 9A through 9C and FIGS. 10A through 10C are cross-sectional viewsillustrating sequential process steps for fabricating a blue lightemitting diode in accordance with the third embodiment of the presentinvention.

First, as shown in FIG. 9A, a low-temperature buffer layer (not shown)is grown on a substrate 11 made of sapphire by a MOVPE process. Thesubstrate 11 is about 5.1 cm in diameter, and the plane orientation ofthe principal surface thereof is a (0001) plane. In the MOVPE process,trimethylgallium is used as a gallium source, ammonia is used as anitrogen source, and hydrogen is used as a carrier gas, while the growthtemperature is set at about 500° C. The buffer layer buffers a latticemismatch between the sapphire and a first semiconductor layer 12, forexample, grown on the sapphire. Subsequently, while mono-silane, whichis a source material containing silicon as a donor impurity, isintroduced, and with the growth temperature being set at about 1030° C.,the first semiconductor layer 12 made of n-type gallium nitride having athickness of about 4 μm is grown on the low-temperature buffer layer.Then, the supply of the mono-silane is stopped, and a barrier layer madeof gallium nitride having a thickness of about 7 nm is grown on thefirst semiconductor layer 12. The carrier gas is then changed tonitrogen (N₂), and at the same time the growth temperature is lowered toabout 800° C., and while trimethylindium as an indium source is alsosupplied, a well layer is grown on the barrier layer. The well layer hasa thickness of about 3 nm and is made of indium gallium nitride, inwhich indium proportion is 30%. The barrier layer and the well layer aregrown alternately in three pairs, thereby forming a MQW active layer 13.Then, cyclopentadienyl magnesium, which is a source material containingmagnesium as an acceptor impurity, is introduced into the respectivesource gases of trimethylgallium and ammonia, and a second semiconductorlayer 14 made of p-type gallium nitride having a thickness of about 0.8μm is grown on the MQW active layer 13. After the second semiconductorlayer 14 has been grown, the second semiconductor layer 14 is subjectedto an annealing process performed for 20 minutes in a nitrogen ambientat a temperature of about 750° C. Through the annealing process, thep-type dopant introduced into the second semiconductor layer 14 isactivated, which further reduces the resistance of the secondsemiconductor layer 14.

Subsequently, as shown in FIG. 9B, a p-type electrode 41 made ofplatinum is formed on the entire surface of the second semiconductorlayer 14 by an EB deposition method, for example. In this process step,the material for the p-type electrode 41 is not limited to platinum, butmay be any material that has excellent ohmic with respect to the p-typesecond semiconductor layer 14, and has high reflectance. For example,rhodium (Rh) or silver (Ag) may be used.

Next, as shown in FIG. 9C, a plated underlying layer 42 made of gold isgrown on the entire surface of the p-type electrode 41 by an EBdeposition method, for example. Then, a plated layer 43 made of goldhaving a thickness of about 50 μm is grown on the entire surface of theplated underlying layer 42 by a plating process.

Thereafter, as shown in FIG. 10A, the substrate 11 is removed from thesemiconductor multilayer structure with the plated layer 43 formedthereon. The substrate 11 may be removed, for example, by a polishingmethod, in which the substrate 11 is polished mechanically, or by alaser lift-off method, in which the substrate 11 is peeled off byirradiating the substrate 11 through its reverse face (that is, its faceopposing the first semiconductor layer 12) with a laser beam having awavelength that passes through sapphire and is absorbed by galliumnitride. In a case of using a laser lift-off method, since metal galliumis produced by the thermal decomposition of the gallium nitride, andadheres to the face of the first semiconductor layer 12 from which thesubstrate 11 has been peeled off, the attached metal gallium has to beremoved using hydrochloric acid.

Subsequently, as shown in FIG. 10B, ITO, into which silicon, i.e., thesame impurity element as the n-type dopant in the first semiconductorlayer 12, has been introduced, is grown to a thickness of about 100 nmby a PLD method, for example, on the exposed face of the firstsemiconductor layer 12, that is, on the face of the first semiconductorlayer 12 opposite to the MQW active layer 13, thereby forming atransparent electrode 45. In this process step, the impurity element tointroduce into the material for the ITO may be a dopant that makes thefirst semiconductor layer 12 develop n-type conductivity. For example,germanium may be used in place of silicon.

Further, the material for the transparent electrode 45 is not limited toITO, but may be any substance that is transparent with respect to lightwith a wavelength of about 470 nm, and tin oxide or zinc oxide may thusbe used.

Furthermore, as in the second embodiment, if the diode is formed so thatthe MQW active layer 13 emits light having a wavelength in theultraviolet region, the use of tin-oxide-added gallium oxide in formingthe transparent electrode 45 makes the transparent electrode 45transparent with respect to the wavelength of the emitted light.

Next, after the transparent electrode 45 has been grown, the transparentelectrode 45 is subjected to an annealing process performed at atemperature of about 500° C. Through the annealing process, part of thesilicon introduced into the ITO is diffused into the first semiconductorlayer 12 through the interface between the transparent electrode 45 andthe first semiconductor layer 12. This results in a decrease in thevalue of resistance in the first semiconductor layer 12 where the firstsemiconductor layer 12 is near the interface with the transparentelectrode 45, thus leading to the formation of the transparent electrode45 with small contact resistance with respect to the first semiconductorlayer 12.

Then, as shown in FIG. 10C, bonding pads 16 for wire bonding areselectively formed on the transparent electrodes 45. The semiconductormultilayer structure is then divided into chips each about 300 μmsquare, thereby obtaining blue light emitting diodes.

As described above, in the blue light emitting diode having the N-upstructure in accordance with the third embodiment, silicon, that is, theimpurity with which the n-type first semiconductor layer 12 has beendoped, is introduced beforehand into the transparent electrode 45(n-type electrode), and then diffused into the first semiconductor layer12 through an annealing process. This allows the transparent electrode45 to have small contact resistance with respect to the firstsemiconductor layer 12, enabling the operating voltage to be decreased.

In addition, the substrate 11 made of sapphire is removed, and theplated layer 43 of gold is formed covering the p-type electrode 41formed on the p-type second semiconductor layer 14. Therefore, when theplated layer 43 is mounted onto a submount, for example, a device havingexcellent heat-dispersion characteristics is obtained.

As a first modified example of the third embodiment, instead ofintroducing a dopant that determines the conductivity type of thegallium nitride into the transparent material (ITO) that forms thetransparent electrode 45, a metal element which tends to adsorb (bindto) hydrogen atoms, e.g., nickel (Ni), palladium (Pd), or platinum (Pt),may be introduced into the transparent material.

Further, as a second modified example, as shown in FIG. 11, the p-typeelectrode 41 made of platinum may be formed to be a transparent p-typeelectrode 41A made of transparent material, and a multilayer film(reflecting film) 46 made of dielectrics or semiconductors may be formedto the side of the transparent p-type electrode 41A opposite to thep-type second semiconductor layer 14.

In this case, it is also preferable that magnesium, which is theacceptor impurity in the second semiconductor layer 14, be introducedinto the transparent p-type electrode 41A. Further, the opticalreflectance of the multilayer film 46 is preferably 70% or higher.

(Fourth Embodiment)

Hereinafter, a fourth embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 12 illustrates a cross-sectional structure of a blue light emittingdiode in accordance with the fourth embodiment of the present invention.In FIG. 12, the same members as those shown in FIGS. 1A and 1B areidentified by the same reference numerals and the description thereofwill be omitted herein.

In the blue light emitting diode in accordance with the fourthembodiment, a passivation film 51 made of magnesium-added silicon oxide(SiO₂) is formed so as to cover the upper surface of a secondsemiconductor layer 14 except the region where a transparent electrode15 is formed, and to cover the respective exposed lateral faces of thesecond semiconductor layer 14, MQW active layer 13, and firstsemiconductor layer 12.

As described above, the lateral sides of the blue light emitting diodeof the fourth embodiment are covered by the passivation film 51, whichprevents current leakage due to the solder material flowing into thelateral sides of the semiconductor multilayer structure when the diodeis mounted onto a submount, for example.

In addition, magnesium, which is the impurity element serving as adopant in the p-type second semiconductor layer 14, is introduced intothe transparent electrode 15 and the passivation film 51. Therefore,contact resistance between the transparent electrode 15 and the secondsemiconductor layer 14 is allowed to be reduced, thereby enablinglow-voltage operation.

It should be noted that the impurity element to introduce into thetransparent electrode 15 is not limited to magnesium, but zinc,beryllium, or any other dopant that makes the conductivity type ofgallium nitride be p-type, may be used.

In addition, instead of the MQW active layer 13, a single-quantum well(SQW) active layer of gallium nitride having a thickness of about 20 nmmay be provided.

Hereinafter, referring to the accompanying drawings, it will bedescribed how to fabricate a blue light emitting diode having theabove-mentioned structure.

FIGS. 13A through 13D and FIGS. 14A through 14C are cross-sectionalviews illustrating sequential process steps for fabricating a blue lightemitting diode in accordance with the fourth embodiment of the presentinvention.

First, as shown in FIG. 13A, a low-temperature buffer layer (not shown)is grown on a substrate 11 made of sapphire by a MOVPE process. Thesubstrate 11 is about 5.1 cm in diameter, and the plane orientation ofthe principal surface thereof is a (0001) plane. In the MOVPE process,trimethylgallium is used as a gallium source, ammonia is used as anitrogen source, and hydrogen is used as a carrier gas, while the growthtemperature is set at about 500° C. The low-temperature buffer layerbuffers a lattice mismatch between the sapphire and a firstsemiconductor layer 12, for example, grown on the sapphire.Subsequently, while mono-silane, which is a source material containingsilicon as a donor impurity, is introduced, and with the growthtemperature being set at about 1030° C., the first semiconductor layer12 made of n-type gallium nitride having a thickness of about 4 μm isgrown on the low-temperature buffer layer. Then, the supply of themono-silane is stopped, and a barrier layer made of gallium nitride witha thickness of about 7 nm is grown on the first semiconductor layer 12.The carrier gas is then changed to nitrogen (N₂), and at the same timethe growth temperature is lowered to about 800° C., and whiletrimethylindium as an indium source is also supplied, a well layer isgrown on the barrier layer. The well layer has a thickness of about 3nm, and is made of indium gallium nitride, in which indium proportion is30%. The barrier layer and the well layer are grown alternately in threepairs, thereby forming a MQW active layer 13. Subsequently,cyclopentadienyl magnesium, which is a source material containingmagnesium as an acceptor impurity, is introduced into the respectivesource gases of trimethylgallium and ammonia, and a second semiconductorlayer 14 made of p-type gallium nitride having a thickness of about 0.8μm is grown on the MQW active layer 13. After the second semiconductorlayer 14 has been grown, the second semiconductor layer 14 is subjectedto an annealing process performed for 20 minutes in a nitrogen ambientat a temperature of about 750° C. Through the annealing process, thep-type dopant introduced into the second semiconductor layer 14 isactivated, which further decreases the resistance of the secondsemiconductor layer 14.

Then, as shown in FIG. 13B, the second semiconductor layer 14, the MQWactive layer 13, and upper portions of the first semiconductor layer 12are selectively removed by dry etching, such as RIE using, e.g.,chlorine as an etching gas, or ICP etching, thereby forming n-typeelectrode formation regions 12 a in the first semiconductor layer 12.

Next, as shown in FIG. 13C, a passivation film 51 made ofmagnesium-added silicon oxide is deposited to a thickness of about 300nm on the entire surface of the second semiconductor layers 14 as wellas on the n-type electrode formation regions 12 a by a sputteringprocess, for example. Subsequently, the passivation film 51 is subjectedto an annealing process performed at a temperature of about 500° C., sothat the magnesium introduced into the passivation film 51 is diffusedfrom the passivation film 51 across the interfaces into the upperportions of the second semiconductor layers 14, thereby decreasing theresistance value of the second semiconductor layers 14 in the vicinityof the interfaces with the passivation film 51. It should be noted thatin order to introduce magnesium into the passivation film 51, if asputtering process is employed, magnesium may be mixed into a targetmaterial, and if a sol-gel method is adopted, magnesium may be mixedinto a source solution as an organic compound.

Next, as shown in FIG. 13D, transparent-electrode formation portions inthe passivation film 51 on the second semiconductor layer 14, andportions of the passivation film 51 located on n-type electrodeformation regions 12 a are selectively removed by dry etching.

Subsequently, as shown in FIG. 14A, ITO, into which magnesium, that is,the impurity element serving as the p-type dopant in the secondsemiconductor layer 14, has been introduced, is selectively grown to athickness of about 100 nm by a sputtering process or a PLD method, forexample, on the exposed faces of the second semiconductor layers 14,thereby forming transparent electrodes 15. Thereafter, the transparentelectrodes 15 are also subjected to an annealing process performed at atemperature of about 500° C. Through the annealing process, themagnesium introduced into the transparent electrodes 15 is furtherdiffused from the transparent electrodes 15 across the interfaces intothe upper portions of the second semiconductor layers 14, therebyfurther decreasing the value of resistance in the second semiconductorlayers 14 where the second semiconductor layers 14 are near theinterfaces with the transparent electrodes 15. This results in theformation of the transparent electrodes 15 having further reducedcontact resistance, on the p-type second semiconductor layers 14.

It should be noted that the impurity element to introduce into thepassivation film 51 and the transparent electrodes 15 is not limited tomagnesium, but may be a dopant, such as zinc, that makes gallium nitridedevelop p-type conductivity.

Then, as shown in FIG. 14B, bonding pads 16 for wire bonding areselectively formed on the respective transparent electrodes 15.Subsequently, titanium and gold are sequentially grown on the n-typeelectrode formation regions 12 a in the first semiconductor layer 12,thereby forming n-type ohmic electrodes 17.

Next, as shown in FIG. 14C, the substrate 11 is divided into chips each300 μm square, thereby obtaining blue light emitting diodes.

As described above, in the blue-light-emitting-diode fabrication methodin accordance with the fourth embodiment, since magnesium, which is thep-type dopant in the second semiconductor layer 14 in contact with thetransparent electrode 15, is introduced into the passivation film 51 andthe transparent electrode 15, the magnesium is diffused into the secondsemiconductor layer 14 through the interfaces with those members by theannealing process performed after the deposition of the passivation film51 and by the annealing process performed after the formation of thetransparent electrode 15 formed after the removal of the passivationfilm 51. As a result, the value of resistance in the secondsemiconductor layer 14 where the second semiconductor layer 14 is in thevicinity of the interface with the transparent electrode 15 decreasessignificantly, such that contact resistance between the transparentelectrode 15 and the second semiconductor layer 14 is reduced, enablingoperation at low operating voltage.

In addition, since the passivation film 51 covers the semiconductormultilayer structure laterally, current leakage, caused by the soldermaterial flowing into the lateral sides of the semiconductor multilayerstructure during mounting process, is prevented, therefore resulting inan increase in yield.

As a first modified example of the fourth embodiment, instead ofintroducing a dopant that determines the conductivity type of thegallium nitride into the passivation film 51 and the transparentmaterial (ITO) that forms the transparent electrode 15, a metal elementwhich tends to adsorb (bind to) hydrogen atoms, e.g., nickel (Ni),palladium (Pd), or platinum (Pt), may be introduced into at least one ofthe passivation film 51 and the transparent material.

Further, the effects of the present invention are obtained byintroducing an impurity that serves as a dopant in the nitridesemiconductor, or a metal element that tends to adsorb hydrogen atoms,into just one of the passivation film 51 and the transparent electrode15.

(Fifth Embodiment)

Hereinafter, a fifth embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 15 illustrates a cross-sectional structure of a blue-lightsurface-emitting laser device in accordance with the fifth embodiment ofthe present invention. In FIG. 15, the same members as those shown inFIGS. 1A and 1B are identified by the same reference numerals and thedescription thereof will be omitted herein.

As shown in FIG. 15, between a substrate 11 made of, e.g., sapphire anda first semiconductor layer 12 made of n-type gallium nitride, a firstDBR (distributed bragg reflect) mirror 61 is formed by alternatelystacking aluminum gallium nitride and gallium nitride one upon theother. Further, a second DBR mirror 65 made of dielectrics is formed inan optical waveguide portion on a transparent electrode 15 that has beendoped with magnesium, for example.

Moreover, a passivation film 51 made of, e.g., magnesium-added siliconoxide is formed on the end portion of a second semiconductor layer 14,thereby forming a current confinement structure for confining operatingcurrent supplied from the transparent electrode 15.

In this embodiment, a MQW active layer 63 made of indium gallium nitridemay have a SQW structure.

By the above-mentioned structure, contact resistance between thetransparent electrode 15 and the p-type second semiconductor layer 14 isdecreased as in the first embodiment, which enables the blue-lightsurface-emitting laser device to operate at low voltage.

In the fifth embodiment, a p-type dopant such as magnesium may beintroduced into at least one of the transparent electrode 15 and thepassivation film 51, but introducing the p-type dopant into both allowsthe effects of the present invention to be attained more notably.

Hereinafter, referring to the accompanying drawings, it will bedescribed how to fabricate a blue-light surface-emitting laser devicehaving the above-mentioned structure.

FIGS. 16A through 16C and FIGS. 17A through 17C are cross-sectionalviews illustrating sequential process steps for fabricating theblue-light surface-emitting laser device in accordance with the fifthembodiment of the present invention.

First, as shown in FIG. 16A, a low-temperature buffer layer (not shown)is grown on a substrate 11 made of sapphire by a MOVPE process. Thesubstrate 11 is about 5.1 cm in diameter, and the plane orientation ofthe principal surface thereof is a (0001) plane. In the MOVPE process,trimethylgallium is used as a gallium source, ammonia is used as anitrogen source, and hydrogen is used as a carrier gas, while the growthtemperature is set at about 500° C. The low-temperature buffer layerbuffers a lattice mismatch between the sapphire and a first DBR mirror61, for example, grown on the sapphire. Subsequently, with the growthtemperature being set at about 1030° C., a first layer made of aluminumgallium nitride and a second layer made of gallium nitride are stackedalternately one upon the other, on the low-temperature buffer layer,thereby forming the first DBR mirror 61. In this process step, ingrowing the first layers, trimethylaluminum as an aluminum source isadded to the source material. The first DBR mirror 61 is formed so as tohave 99% or higher reflectance with respect to the wavelength of lightemitted from a MQW active layer 63. Thereafter, mono-silane, which is asource material containing silicon as a donor impurity, is introduced,and a first semiconductor layer 12 made of n-type gallium nitride isgrown. Then, the supply of the mono-silane is stopped, and a barrierlayer made of gallium nitride is grown on the first semiconductor layer12. The carrier gas is then changed to nitrogen (N₂), and at the sametime the growth temperature is lowered to about 800° C., and whiletrimethylindium (TMI) as an indium source is also supplied, a well layermade of indium gallium nitride is grown on the barrier layer. Thebarrier layer and the well layer are grown, e.g., alternately in threepairs, thereby forming the MQW active layer 63. The MQW active layer 63generates blue light having a wavelength of about 470 nm. Subsequently,cyclopentadienyl magnesium, which is a source material containingmagnesium as an acceptor impurity, is introduced into the respectivesource gases of trimethylgallium and ammonia, and a second semiconductorlayer 14 made of gallium nitride is grown on the MQW active layer 63.After the second semiconductor layer 14 has been grown, the secondsemiconductor layer 14 is subjected to an annealing process performedfor 20 minutes in a nitrogen ambient at a temperature of about 750° C.Through the annealing process, the p-type dopant introduced into thesecond semiconductor layer 14 is activated, which further decreases theresistance of the second semiconductor layer 14.

Then, as shown in FIG. 16B, the second semiconductor layer 14, the MQWactive layer 63, and upper portions of the first semiconductor layer 12are selectively removed by dry etching, such as RIE using, e.g.,chlorine as an etching gas, or ICP etching, thereby forming an n-typeelectrode formation region 12 a in the first semiconductor layer 12.

Next, a passivation film 51 made of magnesium-added silicon oxide isdeposited to a thickness of about 300 nm on the entire surface of thesecond semiconductor layer 14 as well as on the n-type electrodeformation region 12 a by a sputtering process, for example. Thepassivation film 51 is then subjected to an annealing process performedat a temperature of about 500° C., so that the magnesium introduced intothe passivation film 51 is diffused from the passivation film 51 acrossthe interface into the upper portion of the second semiconductor layer14, thereby lowering the resistance value of the second semiconductorlayer 14 in the vicinity of the interface with the passivation film 51.Thereafter, a transparent-electrode formation portion of the passivationfilm 51 on the second semiconductor layer 14, and a portion of thepassivation film 51 located on the n-type electrode formation region 12a are selectively removed by dry etching, thereby resulting in the stateshown in FIG. 16C.

Subsequently, as shown in FIG. 17A, ITO, into which magnesium, i.e., theimpurity element serving as the p-type dopant in the secondsemiconductor layer 14, has been introduced, is selectively grown on theexposed face of the second semiconductor layer 14 by a sputteringprocess or a PLD method, for example, so as to cover the passivationfilm 51, thereby forming a transparent electrode 15. Thereafter, thetransparent electrode 15 is also subjected to an annealing processperformed at a temperature of about 500° C. Through the annealingprocess, the magnesium introduced into the transparent electrode 15 isfurther diffused from the transparent electrode 15 through the interfaceinto the upper portion of the second semiconductor layer 14. Thisfurther reduces the value of resistance in the second semiconductorlayer 14 near the interface with the transparent electrode 15. As aresult, it is possible to form the transparent electrode 15 havingfurther reduced contact resistance, on the p-type second semiconductorlayer 14.

Then, as shown in FIG. 17B, a second DBR mirror 65 is formed by stackinga plurality of dielectric layers having different refraction indexes, onan optical waveguide portion of the transparent electrode 15, that is,on a portion of the transparent electrode 15 which is in contact withthe second semiconductor layer 14.

Next, as shown in FIG. 17C, a bonding pad 16 for wire bonding isselectively formed on the transparent electrode 15 where the transparentelectrode 15 is located on the passivation film 51. Subsequently,titanium and gold are sequentially grown on the n-type electrodeformation region 12 a in the first semiconductor layer 12, therebyforming an n-type ohmic electrode 17.

As described above, in the method for fabricating a blue-lightsurface-emitting laser device in accordance with the fifth embodiment,the passivation film 51 and the transparent electrode 15 are doped withmagnesium that is the p-type dopant in the p-type second semiconductorlayer 14 in contact with the transparent electrode 15. Therefore, themagnesium is diffused into the second semiconductor layer 14 across theinterfaces with the passivation film 51 and the transparent electrode 15by the respective annealing processes performed after the deposition ofthe passivation film 51 and after the formation of the transparentelectrode 15 formed after the removal of the passivation film 51. Thissignificantly reduces the value of resistance in the secondsemiconductor layer 14 where the second semiconductor layer 14 is nearthe interface with the transparent electrode 15, so that contactresistance between the transparent electrode 15 and the secondsemiconductor layer 14 is decreased, which enables operation at lowoperating voltage.

As a first modified example of the fifth embodiment, instead ofintroducing a dopant that determines the conductivity type of thegallium nitride into the passivation film 51 and the transparentmaterial (ITO) that forms the transparent electrode 15, a metal elementwhich tends to adsorb (bind to) hydrogen atoms, e.g., nickel (Ni),palladium (Pd), or platinum (Pt), may be introduced into at least one ofthe passivation film 51 and the transparent material.

Further, the effects of the present invention are obtained byintroducing an impurity that serves as a dopant in the nitridesemiconductor, or a metal element that tends to adsorb hydrogen atoms,into just one of the passivation film 51 and the transparent electrode15.

Moreover, as a second modified example, the substrate 11 may be removedas in the third embodiment, and an n-type ohmic electrode may be formedon the face of the first DBR mirror 61 opposite to the firstsemiconductor layer 12. Further, in the case of removing the substrate11, the first DBR mirror 61 may be etched so that part of the firstsemiconductor layer 12 is exposed, and an n-type ohmic electrode may beformed on the exposed portion. Furthermore, in the case of removing thesubstrate 11, instead of forming the first DBR mirror 61 made of thenitride semiconductors in the process steps for growing thesemiconductor multilayer structure, a first DBR mirror 61 made ofdielectric materials instead of the nitride semiconductors, may beformed on the face of the first semiconductor layer 12 opposite to theMQW active layer 63, after the substrate 11 has been removed.

In a case in which the first and second DBR mirrors 61 and 65 are formedout of dielectric materials, among silicon oxide (SiO₂), silicon nitride(Si₃N₄), niobium oxide (Nb₂O₅), hafnium oxide (HfO₂), titanium oxide(TiO₂) and tantalum oxide (Ta₂O₅), at least two substances havingdifferent refraction indexes may be selected as the dielectricmaterials.

Further, in the first through fifth embodiments, although the principalsurface of the substrate 11 is not processed at all, a mask forselective growth may be formed on the substrate 11, or steps may becreated in the upper portion of the substrate 11, so that a selectiveepitaxial lateral over growth (ELOG) may be performed.

Furthermore, in the foregoing embodiments, the material for thesubstrate is not limited to sapphire, but silicon carbide (SiC), galliumarsenide (GaAs), zinc oxide (ZnO), spinel, or silicon (Si), for example,may be used.

Moreover, in the foregoing embodiments, although light emitting diodesand surface-emitting laser devices are described as surface-emittingnitride semiconductor light-emitting devices, any semiconductor lightemitting devices, in which a transparent electrode is provided on anitride semiconductor, produce the effects of the present invention.

1. A semiconductor light emitting device comprising: a semiconductormultilayer structure comprising a plurality of Group III–V nitridesemiconductor layers including two semiconductor layers of differentconductivity types, and a transparent electrode formed on thesemiconductor multilayer structure, wherein the transparent electrodecontains an impurity element developing the same conductivity type asthat of an impurity element introduced into a semiconductor in thesemiconductor multilayer structure, said semiconductor having aninterface with the transparent electrode, and the impurity elementcontained in the transparent electrode is diffused into thesemiconductor having the interface with the transparent electrode,causing contact resistance between the semiconductor and the transparentelectrode to decrease.
 2. The semiconductor light emitting device ofclaim 1, wherein the impurity elements are magnesium, zinc, beryllium,or silicon.
 3. The semiconductor light emitting device of claim 1,wherein the transparent electrode is made of indium tin oxide or galliumoxide.
 4. The semiconductor light emitting device of claim 1, furthercomprising, on the transparent electrode, a multilayer film thatreflects light emitted from the semiconductor multilayer structure, andincludes a plurality of dielectric layers.
 5. The semiconductor lightemitting device of claim 4, wherein the multilayer film is made of atleast two substances among silicon oxide, silicon nitride, niobiumoxide, hafnium oxide, titanium oxide and tantalum oxide.
 6. Thesemiconductor light emitting device of claim 1, further comprising: amultilayer film, which is formed to the side of the semiconductormultilayer structure opposite to the transparent electrode, and whichreflects light emitted from the semiconductor multilayer structure, andincludes a plurality of dielectric layers or a plurality ofsemiconductor layers.
 7. The semiconductor light emitting device ofclaim 6, wherein the multilayer film is made of at least two substancesamong silicon oxide, silicon nitride, niobium oxide, hafnium oxide,titanium oxide and tantalum oxide.